Sex Chromosome System Research Articles

Pistachio genomes provide insights into nut tree domestication and ZW sex chromosome evolution

Pistachio is a nut crop domesticated in the Fertile Crescent and a dioecious species with ZW sex chromosomes. We sequenced the genomes of Pistacia vera cultivar (cv.) Siirt, the female parent, and P. vera cv. Bagyolu, the male parent. Two chromosome-level reference genomes of pistachio were generated, and Z and W chromosomes were assembled. The ZW chromosomes originated from an autosome following the first inversion, which occurred approximately 8.18 Mya. Three inversion events in the W chromosome led to the formation of a 12.7-Mb (22.8% of the W chromosome) non-recombining region. These W-specific sequences contain several genes of interest that may have played a pivotal role in sex determination and contributed to the initiation and evolution of a ZW sex chromosome system in pistachio. The W-specific genes, including defA, defA-like, DYT1, two PTEN1, and two tandem duplications of six VPS13A paralogs, are strong candidates for sex determination or differentiation. Demographic history analysis of resequenced genomes suggest that cultivated pistachio underwent severe domestication bottlenecks approximately 7640 years ago, dating the domestication event close to the archeological record of pistachio domestication in Iran. We identified 390, 211, and 290 potential selective sweeps in 3 cultivar subgroups that underlie agronomic traits such as nut development and quality, grafting success, flowering time shift, and drought tolerance. These findings have improved our understanding of the genomic basis of sex determination/differentiation and horticulturally important traits and will accelerate the improvement of pistachio cultivars and rootstocks.

Karyotype differentiation and male meiosis in European clades of the spider genus Pholcus (Araneae, Pholcidae).

Haplogyne araneomorphs are a diverse spider clade. Their karyotypes are usually predominated by biarmed (i.e., metacentric and submetacentric) chromosomes and have a specific sex chromosome system, X1X2Y. These features are probably ancestral for haplogynes. Nucleolus organizer regions (NORs) spread frequently from autosomes to sex chromosomes in these spiders. This study focuses on pholcids (Pholcidae), a highly diverse haplogyne family. Despite considerable recent progress in pholcid cytogenetics, knowledge on many clades remains insufficient including the most species-rich pholcid genus, Pholcus Walckenaer, 1805. To characterize the karyotype differentiation of Pholcus in Europe, we compared karyotypes, sex chromosomes, NORs, and male meiosis of seven species [P.alticeps Spassky, 1932; P.creticus Senglet, 1971; P.dentatus Wunderlich, 1995; P.fuerteventurensis Wunderlich, 1992; P.phalangioides (Fuesslin, 1775); P.opilionoides (Schrank, 1781); P.silvai Wunderlich, 1995] representing the dominant species groups in this region. The species studied show several features ancestral for Pholcus, namely the 2n♂ = 25, the X1X2Y system, and a karyotype predominated by biarmed chromosomes. Most taxa have a large acrocentric NOR-bearing pair, which evolved from a biarmed pair by a pericentric inversion. In some lineages, the acrocentric pair reverted to biarmed. Closely related species often differ in the morphology of some chromosome pairs, probably resulting from pericentric inversions and/or translocations. Such rearrangements have been implicated in the formation of reproductive barriers. While the X1 and Y chromosomes retain their ancestral metacentric morphology, the X2 chromosome shows a derived (acrocentric or subtelocentric) morphology. Pairing of this element is usually modified during male meiosis. NOR patterns are very diverse. The ancestral karyotype of Pholcus contained five or six terminal NORs including three X chromosome-linked loci. The number of NORs has been frequently reduced during evolution. In the Macaronesian clade, there is only a single NOR-bearing pair. Sex chromosome-linked NORs are lost in Madeiran species and in P.creticus. Our study revealed two cytotypes in the synanthropic species P.phalangioides (Madeiran and Czech), which differ by their NOR pattern and chromosome morphology. In the Czech cytotype, the large acrocentric pair was transformed into a biarmed pair by pericentric inversion.

A step forward in the genome characterization of the sugarcane borer, Diatraea saccharalis: karyotype analysis, sex chromosome system and repetitive DNAs through a cytogenomic approach.

Moths of the family Crambidae include a number of pests that cause economic losses to agricultural crops. Despite their economic importance, little is known about their genome architecture and chromosome evolution. Here, we characterized the chromosomes and repetitive DNA of the sugarcane borer Diatraea saccharalis using a combination of low-pass genome sequencing, bioinformatics, and cytogenetic methods, focusing on the sex chromosomes. Diploid chromosome numbers differed between the sexes, i.e., 2n = 33 in females and 2n = 34 in males. This difference was caused by the occurrence of a WZ1Z2 trivalent in female meiosis, indicating a multiple sex-chromosome system WZ1Z2/Z1Z1Z2Z2. A strong interstitial telomeric signal was observed on the W chromosome, indicating a fusion of the ancestral W chromosome with an autosome. Among repetitive DNAs, transposable elements (TEs) accounted for 39.18% (males) to 41.35% (females), while satDNAs accounted for only 0.214% (males) and 0.215% (females) of the genome. FISH mapping revealed different chromosomal organization of satDNAs, such as single localized clusters, spread repeats, and non-clustered repeats. Two TEs mapped by FISH were scattered. Although we found a slight enrichment of some satDNAs in the female genome, they were not differentially enriched on the W chromosome. However, we found enriched FISH signals for TEs on the W chromosome, suggesting their involvement in W chromosome degeneration and differentiation. These data shed light on karyotype and repetitive DNA dynamics due to multiple chromosome fusions in D. saccharalis, contribute to the understanding of genome structure in Lepidoptera and are important for future genomic studies.

Sex chromosome differentiation via changes in the Y chromosome repeat landscape in African annual killifishes Nothobranchius furzeri and N. kadleci.

Homomorphic sex chromosomes and their turnover are common in teleosts. We investigated the evolution of nascent sex chromosomes in several populations of two sister species of African annual killifishes, Nothobranchius furzeri and N. kadleci, focusing on their under-studied repetitive landscape. We combined bioinformatic analyses of the repeatome with molecular cytogenetic techniques, including comparative genomic hybridization, fluorescence in situ hybridization with satellite sequences, ribosomal RNA genes (rDNA) and bacterial artificial chromosomes (BACs), and immunostaining of SYCP3 and MLH1 proteins to mark lateral elements of synaptonemal complexes and recombination sites, respectively. Both species share the same heteromorphic XY sex chromosome system, which thus evolved prior to their divergence. This was corroborated by sequence analysis of a putative master sex determining (MSD) gene gdf6Y in both species. Based on their divergence, differentiation of the XY sex chromosome pair started approximately 2 million years ago. In all populations, the gdf6Y gene mapped within a region rich in satellite DNA on the Y chromosome long arms. Despite their heteromorphism, X and Y chromosomes mostly pair regularly in meiosis, implying synaptic adjustment. In N. kadleci, Y-linked paracentric inversions like those previously reported in N. furzeri were detected. An inversion involving the MSD gene may suppress occasional recombination in the region, which we otherwise evidenced in the N. furzeri population MZCS-121 of the Limpopo clade lacking this inversion. Y chromosome centromeric repeats were reduced compared with the X chromosome and autosomes, which points to a role of relaxed meiotic drive in shaping the Y chromosome repeat landscape. We speculate that the recombination rate between sex chromosomes was reduced due to heterochiasmy. The observed differences between the repeat accumulations on the X and Y chromosomes probably result from high repeat turnover and may not relate closely to the divergence inferred from earlier SNP analyses.

First Insights on the Karyotype Diversification of the Endemic Malagasy Leaf-Toed Geckos (Squamata: Gekkonidae: Uroplatus).

Simple SummaryThe geckos of the genus Uroplatus include peculiar endemic species to Madagascar. Even though they have been the subject of several morphological and molecular studies, karyological analyses have been performed only on U. phantasticus, leaving the chromosomal diversity of the genus completely unexplored. In this study, we performed a preliminary molecular analysis and a comparative cytogenetic study providing the first karyotype description of eight species of Uroplatus and an assessment of their karyological variability. We found chromosome diversity in the species studied in terms of total chromosome number (2n = 34–38), localization of loci of Nucleolar Organizer Regions (NORs) (alternatively on the 2nd, 6th, 10th or 16th pair), heterochromatin composition and occurrence of heteromorphic sex chromosome pairs. Adding our newly generated data to those available from the literature, we show that in the genus Uroplatus, as well as in a larger group of phylogenetically related gecko genera, chromosome diversification mainly occurred toward a reduction in the chromosome number by means of chromosome fusions and translocation of NOR-bearing chromosomes. We also hypothesize that the diversification of sex chromosome systems occurred independently in different genera.We provide here the first karyotype description of eight Uroplatus species and a characterization of their chromosomal diversity. We performed a molecular taxonomic assessment of several Uroplatus samples using the mitochondrial 12S marker and a comparative cytogenetic analysis with standard karyotyping, silver staining (Ag-NOR) and sequential C-banding + Giemsa, +Chromomycin A3 (CMA3), +4′,6-diamidino-2-phenylindole (DAPI). We found chromosomal variability in terms of chromosome number (2n = 34–38), heterochromatin composition and number and localization of loci or Nucleolar Organizer Regions (NORs) (alternatively on the 2nd, 6th, 10th or 16th pair). Chromosome morphology is almost constant, with karyotypes composed of acrocentric chromosomes, gradually decreasing in length. C-banding evidenced a general low content of heterochromatin, mostly localized on pericentromeric and telomeric regions. Centromeric bands varied among the species studied, resulting in CMA3 positive and DAPI negative or positive to both fluorochromes. We also provide evidence of a first putative heteromorphic sex chromosome system in the genus. In fact, in U. alluaudi the 10th pair was highly heteromorphic, with a metacentric, largely heterochromatic W chromosome, which was much bigger than the Z. We propose an evolutionary scenario of chromosome reduction from 2n = 38 to 2n = 34, by means of translocations of microchromosomes on larger chromosomes (often involving the NOR-bearing microchromosomes). Adding our data to those available from the literature, we show that similar processes characterized the evolutionary radiation of a larger gecko clade. Finally, we hypothesize that sex chromosome diversification occurred independently in different genera.

Highly rapid and diverse sex chromosome evolution in the Odorrana frog species complex.

Sex chromosomes in poikilothermal vertebrates are characterized by rapid and diverse evolution at the species or population level. Our previous study revealed that the Taiwanese frog Odorrana swinhoana (2n=26) has a unique system of multiple sex chromosomes created by three sequential translocations among chromosomes 1, 3, and 7. To reveal the evolutionary history of sex chromosomes in the Odorrana species complex, we first identified the original, homomorphic sex chromosomes, prior to the occurrence of translocations, in the ancestral-type population of O. swinhoana. Then, we extended the investigation to a closely related Japanese species, Odorrana utsunomiyaorum, which is distributed on two small islands. We used a high-throughput nuclear genomic approach to analyze single-nucleotide polymorphisms and identify the sex-linked markers. Those isolated from the O. swinhoana ancestral-type population were found to be aligned to chromosome 1 and showed male heterogamety. In contrast, almost all the sex-linked markers isolated from O. utsunomiyaorum were heterozygous in females and homozygous in males and were aligned to chromosome 9. Morphologically, we confirmed chromosome 9 to be heteromorphic in females, showing a ZZ-ZW sex determination system, in which the W chromosomes were heterochromatinized in a stripe pattern along the chromosome axis. These results indicated that after divergence of the two species, the ancestral homomorphic sex chromosome 1 underwent highly rapid and diverse evolution, i.e., sequential translocations with two autosomes in O. swinhoana, and turnover to chromosome 9 in O. utsunomiyaorum, with a transition from XY to ZW heterogamety and change to heteromorphy.

W Chromosome Evolution by Repeated Recycling in the Frog Glandirana rugosa

The Y or W sex chromosome of a heteromorphic pair is usually heterochromatinised and degenerated. However, whether chromosome degeneration constantly proceeds toward an extreme end is not fully understood. Here, we present a case of intermittent evolution of W chromosomes caused by interpopulation hybridisation in the Japanese soil-frog, Glandirana rugosa. This species includes two heteromorphic sex chromosome systems, which are separated into geographic populations, namely the XY and ZW groups. In this study, to uncover the evolutionary mechanisms of the heterogeneous W chromosomes, we genetically investigated the geographic differentiation of the ZW populations along with the closely located XY populations. Analysis of mitochondrial cytochrome b sequences detected three distinct clades, named ZW1, ZW2, and ZW3. High throughput analyses of nuclear genomic DNA showed that autosomal alleles of XY populations were deeply introgressed into the ZW3 sub-group. Based on the genotypes of sex-linked single nucleotide polymorphisms, W-borne androgen receptor gene expression, and WW developmental mortality, we concluded that the X chromosomes were recycled to W chromosomes. Upon inclusion of two cases from another group, Neo-ZW, we observed that the X chromosomes were recycled independently at least four times to the new W chromosomes: a repetition of degeneration and resurrection.

Sex Determination in Plants and its Contribution to Genetic Variability

Sex determination is a process that leads to the physical separation of male and female gamete-producing structures to different individuals of a species. Sexual reproduction is an ancient feature of eukaryote life, yet the sexes as we currently recognize them are relative late comers in the evolution of sex. Sex determination systems in plants have evolved many times from hermaphroditic ancestors (including monoecious plants with separate male and female flowers on the same individual),and sex chromosome systems have arisen several times in flowering plant evolution. Sex chromosome evolution is intimately connected with Y chromosome degeneration. Most current understanding of how the distinctive properties of Y chromosomes evolved comes from theoretical work on the evolution of genomic regions with low recombination. The identification of sex chromosomes in plants is problematic because most of them do not differ morphologically from autosomes or from one another. For example in some species, such as Actinidia deliciosa var. deliciosa, X and Y chromosomes are too small to support observations of their distinguishing characteristics.) In the majority of plants, male and female organs are formed and developed simultaneously, but only up to a point when the growth of either set of sex organs is inhibited. Inhibition of sexual development can vary in character so that in most cases, sexual development is inhibited by the absence of cell division. In many species of bryophytes, heterothallism (unisexuality) has been correlated with the presence of sex chromosomes. Although the extent of heterothallism and sex chromosomes in the bryophytes has not been assessed systematically, this is the only known group of homosporous plants that uses sex chromosomes in sex determination. To date, studies of bryophyte sex determination have focused on the heterothallic liverwort Marchantia polymorpha. Many dioecious species, including those with well-developed sex chromosomes, ........

Identification of a candidate sex determination gene in Culaea inconstans suggests convergent recruitment of an Amh duplicate in two lineages of stickleback.

Sex chromosomes vary greatly in their age and levels of differentiation across the tree of life. This variation is largely due to the rates of sex chromosome turnover in different lineages; however, we still lack an explanation for why sex chromosomes are so conserved in some lineages (e.g. mammals, birds) but so labile in others (e.g. teleosts, amphibians). To identify general mechanisms driving transitions in sex determination systems or forces which favour their conservation, we first require empirical data on sex chromosome systems from multiple lineages. Stickleback fishes are a valuable model lineage for the study of sex chromosome evolution due to variation in sex chromosome systems between closely-related species. Here, we identify the sex chromosome and a strong candidate for the master sex determination gene in the brook stickleback, Culaea inconstans. Using whole-genome sequencing of wild-caught samples and a lab cross, we identify AmhY, a male specific duplication of the gene Amh, as the candidate master sex determination gene. AmhY resides on Chromosome 20 in C. inconstans and is likely a recent duplication, as both AmhY and the sex-linked region of Chromosome 20 show little sequence divergence. Importantly, this duplicate AmhY represents the second independent duplication and recruitment of Amh as the sex determination gene in stickleback and the eighth example known across teleosts. We discuss this convergence in the context of sex chromosome turnovers and the role that the Amh/AmhrII pathway, which is crucial for sex determination, may play in the evolution of sex chromosomes in teleosts.

Atypus karschi Dönitz, 1887 (Araneae: Atypidae): An Asian purse-web spider established in Pennsylvania, USA

The mygalomorph spiders of the family Atypidae are among the most archaic spiders. The genus Atypus Latreille, 1804 occurs in Eurasia and northern Africa, with a single enigmatic species, Atypus snetsingeri Sarno, 1973, known only from a small area in southeastern Pennsylvania in eastern USA. A close relationship to European species could be assumed based on geographic proximity, but A. snetsingeri more closely resembled Asian species. This study was undertaken to learn more about the genetics of A. snetsingeri, its habitat requirements and natural history. Molecular markers (CO1 sequences) were compared to available data for other atypids and showed that A. snetsingeri is identical with A. karschi Dönitz, 1887 native to East Asia. Natural history parameters in Pennsylvania were also similar in every respect to A. karschi in Japan, therefore, we propose that the spider is an introduced species and the specific epithet snetsingeri is relegated to a junior synonym of A. karschi. Cytogenetic analysis showed an X0 sex chromosome system (42 chromosomes in females, 41 in males) and we also detected nucleolus organizing regions and heterochromatin, the latter for the first time in the Atypoidea. In Pennsylvania the spider is found in a variety of habitats, from forests to suburban shrubbery, where the above-ground webs are usually attached vertically to trees, shrubs, or walls, although other webs are oriented horizontally near the ground. Prey include millipedes, snails, woodlice, carabid beetles and earthworms. Atypus karschi is the first known case of an introduced purse-web spider. It is rarely noticed but well-established within its range in southeastern Pennsylvania.

Comparative Cytogenetics of Lace Bugs (Tingidae, Heteroptera): New Data and a Brief Overview.

Simple SummaryTingidae, or lace bugs, is a family of herbivorous true bugs, with approximately 2600 identified species in 318 genera classified in two or sometimes three subfamilies, among which the largest subfamily Tinginae comprises about 2500 species. Here, an account is given of the karyotypes of 16 lace bug species studied using conventional chromosomal techniques and FISH with two repetitive DNA probes, 18S rDNA and (TTAGG)n. We also summarize and analyze all information accumulated to date on karyotypes of lace bugs. In general, such information is available for 60 species and 22 genera of Tinginae. We show that lace bugs are characterized by: (1) very conservative karyotypes, with six pairs of autosomes in a haploid set; (2) either an XY or X(0) sex chromosome system; (3) a conventional sequence of sex chromosome divisions in male meiosis; (4) absence of the (TTAGG)n mechanism for maintaining telomere integrity; (5) four discrete patterns of 18S rDNA chromosomal localization. We conclude that the search for chromosomal landmarks is of paramount importance for the characterization of the lace bug cytogenetics in more detail.The lace bug family Tingidae comprises more than 2600 described species in 318 genera that are classified into the subfamilies Tinginae (about 2500 species and 300 genera), Cantacaderinae, and Vianadinae. We provide data on karyotypes of 16 species belonging to 10 genera of the tribes Tingini and Acalyptaini (Tinginae) studied using conventional chromosome staining and FISH. The species of Tingini possess 2n = 12A + XY, whereas those of Acalyptaini have 2n = 12A + X(0). FISH for 18S rDNA revealed hybridization signals on one of the medium-sized bivalents in species of both tribes. FISH with a telomeric probe TTAGG produced no signals in any species. In addition, we provide a list of all data obtained to date on Tingidae karyotypes, which includes 60 species from 22 genera of Tinginae. The subfamily is highly conservative in relation to the number and size of autosomes, whereas it shows diversity in the number and chromosomal distribution of the rDNA arrays, which may be located either on a pair of autosomes (the predominant and supposedly ancestral pattern), on one or both sex chromosomes, or on an autosome pair and the X. The absence of the “insect” telomeric sequence TTAGG in all species implies that Tinginae have some other, yet unknown, telomere organization.

ESTUDIO CITOGENÉTICO EN ARAÑAS DE ARENA (SICARIIDAE) DE LA CAATINGA BRASILEÑA: DIVERSIDAD DEL SISTEMA DE CROMOSOMAS SEXUALES EN ESPECIES ESTRECHAMENTE RELACIONADAS

In this study, we investigated the chromosomes of three species of Sicarius spiders from the Brazilian Caatinga, using classical and molecular cytogenetic techniques. Based on the phylogenetic approach, we also discussed about the variation of diploid number, types of sex chromosome system and changes in the localization of ribosomal genes of Scytodoidea. Sicarius are Synspermiata spiders that together with the genera Loxosceles and Hexophthalma constitute the family Sicariidae. In this group, the available cytogenetic data showed a low diploid number range (2n♂=18 to 2n♂=23) and the presence of only multiple sex chromosome systems (X1X2Y and X1X20). Mitotic metaphase cells exhibited 2n♂=16+X1X2Y for Sicarius cariri and S. ornatus, and 2n♂=18+XY for S. tropicus. In these species, silver impregnation revealed nucleolar organizer region (Ag-NOR) on the terminal region of pair 1. In S. ornatus and S. tropicus, the results obtained with fluorescent in situ hybridization (FISH) using 18S rDNA probe were similar to Ag-NOR, however in S. cariri, the ribosomal sites were localized in the terminal region of the X1 sex chromosome. In this work, we presented the first description of a simple sex chromosome system for Sicariidae, helping to understand how the XY sex chromosome system evolved from the X1X2Y system. Additionally, FISH data incongruous with Ag-NOR indicate that the cytogenetic studies in Sicariidae allow investigating the relation between the karyotype evolution and the distribution and the activity of rDNA genes. Key words: karyotype, mitosis, nucleolar organizer region, rDNA, Sicarius

Physiological aspects of sex differences and Haldane’s rule in Rumex hastatulus

Haldane’s rule (HR, impairment of fertility and/or viability of interracial hybrids) seems to be one of few generalizations in evolutionary biology. The validity of HR has been confirmed in animals, and more recently in some dioecious plants (Silene and Rumex). Dioecious Rumex hastatulus has two races differing in the sex chromosome system: Texas (T) and North Carolina (NC), and T × NC males showed both reduced pollen fertility and rarity—two classical symptoms of Haldane’s rule (HR). The reduced fertility of these plants has a simple mechanistic explanation, but the reason for their rarity was not elucidated. Here, we measured selected physiological parameters related to the antioxidant defense system in parental races and reciprocal hybrids of R. hastatulus. We showed that the X-autosome configurations, as well as asymmetries associated with Y chromosomes and cytoplasm, could modulate this system in hybrids. The levels and quantitative patterns of the measured parameters distinguish the T × NC hybrid from the other analyzed forms. Our observations suggest that the rarity of T × NC males is caused postzygotically and most likely related to the higher level of oxidative stress induced by the chromosomal incompatibilities. It is the first report on the physiological aspects of HR in plants.

Sex chromosome systems in Neotropical Primates: What have we learnt so far from cytogenetics and genomics?

Neotropical Primates (Platyrrhini) show great diversity in their life histories, ecology, behaviour and genetics. This diversity extends to their chromosome complements, both to autosomes and to sex chromosomes. In this contribution, we will review what is currently known about sex chromosomes in this group, both from cytogenetic and from genomic evidence. The X and Y chromosomes in Neotropical Primates, also known as New World Monkeys, have striking structural differences compared with Old World Monkeys when Catarrhini sex chromosomes are considered. The XY bivalent displays a different meiotic behaviour in prophase I, and their Y chromosome shows extensive genomic differences. Even though the most widespread sex chromosome system is the XX/XY and thus considered the ancestral one for Platyrrhini, modifications of this sexual system are observed within this group. Multiple sex chromosome systems originated from Y-autosome translocations were described in several genera (Aotus, Callimico and Alouatta). In the howler monkeys, genus Alouatta, an independent origin of the sexual systems in South American and Mesoamerican species was postulated. All the above-mentioned evidence suggests that the Y chromosome of Platyrrhini has a different evolutionary history compared with the Catarrhini Y. There is still much to understand regarding their sex chromosome systems.

Sex chromosomes as supergenes of speciation: why amphibians defy the rules?

As reflected by the two rules of speciation (Haldane's rule and the large X-/Z-effect), sex chromosomes are expected to behave like supergenes of speciation: they recombine only in one sex (XX females or ZZ males), supposedly recruit sexually antagonistic genes and evolve faster than autosomes, which can all contribute to pre-zygotic and post-zygotic isolation. While this has been mainly studied in organisms with conserved sex-determining systems and highly differentiated (heteromorphic) sex chromosomes like mammals, birds and some insects, these expectations are less clear in organismal groups where sex chromosomes repeatedly change and remain mostly homomorphic, like amphibians. In this article, we review the proposed roles of sex-linked genes in isolating nascent lineages throughout the speciation continuum and discuss their support in amphibians given current knowledge of sex chromosome evolution and speciation modes. Given their frequent recombination and lack of differentiation, we argue that amphibian sex chromosomes are not expected to become supergenes of speciation, which is reflected by the rarity of empirical studies consistent with a 'large sex chromosome effect' in frogs and toads. The diversity of sex chromosome systems in amphibians has a high potential to disentangle the evolutionary mechanisms responsible for the emergence of sex-linked speciation genes in other organisms. This article is part of the theme issue 'Genomic architecture of supergenes: causes and evolutionary consequences'.

Identification of ancestral sex chromosomes in the frog Glandirana rugosa bearing XX-XY and ZZ-ZW sex-determining systems.

Sex chromosomes constantly exist in a dynamic state of evolution: rapid turnover and change of heterogametic sex during homomorphic state, and often stepping out to a heteromorphic state followed by chromosomal decaying. However, the forces driving these different trajectories of sex chromosome evolution are still unclear. The Japanese frog Glandirana rugosa is one taxon well suited to the study on these driving forces. The species has two different heteromorphic sex chromosome systems, XX-XY and ZZ-ZW, which are separated in different geographic populations. Both XX-XY and ZZ-ZW sex chromosomes are represented by chromosome 7 (2n=26). Phylogenetically, these two systems arose via hybridization between two ancestral lineages of West Japan and East Japan populations, of which sex chromosomes are homomorphic in both sexes and to date have not yet been identified. Identification of the sex chromosomes will give us important insight into the mechanisms of sex chromosome evolution in this species. Here, we used a high-throughput genomic approach to identify the homomorphic XX-XY sex chromosomes in both ancestral populations. Sex-linked DNA markers of West Japan were aligned to chromosome 1, whereas those of East Japan were aligned to chromosome 3. These results reveal that at least two turnovers across three different sex chromosomes 1, 3 and 7 occurred during evolution of this species. This finding raises the possibility that cohabitation of the two different sex chromosomes from ancestral lineages induced turnover to another new one in their hybrids, involving transition of heterogametic sex and evolution from homomorphy to heteromorphy.

Identification of sex chromosomes using genomic and cytogenetic methods in a range-expanding spider,Argiope bruennichi(Araneae: Araneidae)

AbstractDifferences between sexes in growth, ecology and behaviour strongly shape species biology. In some animal groups, such as spiders, it is difficult or impossible to identify the sex of juveniles based on external morphology. This information would be useful for field surveys, behavioural experiments and ecological studies, such as those on sex ratios and dispersal. In species with sex chromosomes, sex can be determined based on the specific sex chromosome complement. Additionally, information on the sequence of sex chromosomes provides the basis for studying sex chromosome evolution. We combined cytogenetic and genomic data to identify the sex chromosomes in the sexually dimorphic spider Argiope bruennichi and designed quantitative real-time polymerase chain reaction sex markers. We found that the genome size and GC content of this spider fall into the ranges reported for the majority of araneids. The male karyotype is formed by 24 acrocentric chromosomes with an X1X20 sex chromosome system, with little similarity between X chromosomes, suggesting an origin of these chromosomes by X chromosome fission or early duplication of an X chromosome and subsequent independent differentiation of the copies. Our data suggest X chromosomes of similar sizes in A. bruennichi. They are smaller chromosomes of the complement. Our findings open the door to new directions in spider evolutionary and ecological research.

GISH painting of the Y chromosomes suggests advanced phases of sex chromosome evolution in three dioecious Cannabaceae species (Humulus lupulus, H. japonicus, and Cannabis sativa).

In plants, dioecy is relatively rare, and it involves sex chromosome systems that often developed independently over time. These characteristics make dioecious plants an attractive model to study sex chromosome evolution. To clarify the patterns of plant sex chromosome evolution, studies should be performed on a wide range of dioecious species. It is interesting to study the sex chromosomes in related species that evolved during a long period of independent sex chromosome evolution. The Cannabaceae family includes three dioecious species with heteromorphic sex chromosomes. Cannabis sativa and Humulus lupulus use the XX/XY chromosome system, whereas Humulus japonicus contains multiple sex chromosomes (XX/XY1Y2). To better understand sex chromosome evolution and the level of genomic divergence of these three related species, we undertook self-GISH and comparative GISH analyses. The self-GISH allowed visualization of the Y chromosomes of C. sativa, H. lupulus, and H. japonicus. The self-GISH signal was distributed along the entire Y chromosome, excluding the pseudo-autosomal region (PAR). Our results indicate that the male-specific region of the Y chromosome (MSY) spans the overwhelming majority of the Y chromosomes of all three species studied. The self-GISH results reveal the accumulation of repetitive DNA sequences in the Y chromosomes of all three species studied. This sequences presented in autosomes and/or chromosome X at a lower copy number than in Y. In comparative GISH experiments where the probe DNA of one species was applied to another species, a weak signal was exclusively detected on 45S rDNA sites, indicating a high level of genomic differentiation of the species used in this study. We demonstrate small PAR size and opposing large MSY and its positions on Y chromosomes. We also found that these genomes are highly differentiated. Furthermore, the data obtained in this study indicate a long period of independent and advanced sex chromosome evolution. Our study provides a valuable basis for future genomic studies of sex and suggests that the Cannabaceae family offers a promising model to study sex chromosome evolution.

Chromosomal distribution of major rDNA and genome size variation in Belostoma angustum Lauck, B. nessimiani Ribeiro & Alecrim, and B. sanctulum Montandon (Insecta, Heteroptera, Belostomatidae).

Known as "electric-light bugs", belostomatids potentially act as agents of biological control. The Belostoma genus has holokinetic chromosomes, interspecific variation in diploid number, sex chromosome system and DNA content. Thus, the chromosomal complement, the accumulation of constitutive heterochromatin and the distribution of rDNA clusters by fluorescence in situ hybridization (FISH) in Belostoma angustum (BAN), Belostoma sanctulum (BSA), and Belostoma nessimiani (BNE) were evaluated. In addition, a comparative analysis of the DNA content of these species and B. estevezae (BES) was performed. BES has the highest Belostoma DNA content, while BSA has the lowest. BAN showed 2n = 29 + X1X2Y, while BSA and BNE had 2n = 14 + XY. BSA showed 18S rDNA markings on sex chromosomes, while BNE and BAN did on autosomes. The difference between BSA and BNE occurs because of the possible movement of the rDNA cluster in BNE. We suggest the occurrence of fusion in the autosomes of BSA and BNE, and fragmentation in the sex chromosomes in BAN. Also, the genome size of 1-2pg represents a haploid DNA content of acommon ancestor, fromwhich the genomes of BES and BAN had evolved by gene duplication and heterochromatinization events.

Neo-sex chromosome evolution shapes sex-dependent asymmetrical introgression barrier

Sex chromosomes play a special role in the evolution of reproductive barriers between species. Here we describe conflicting roles of nascent sex chromosomes on patterns of introgression in an experimental hybrid swarm. Drosophila nasuta and Drosophila albomicans are recently diverged, fully fertile sister species that have different sex chromosome systems. The fusion between an autosome (Muller CD) with the ancestral X and Y gave rise to neo-sex chromosomes in D. albomicans, while Muller CD remains unfused in D. nasuta. We found that a large block containing overlapping inversions on the neo-sex chromosome stood out as the strongest barrier to introgression. Intriguingly, the neo-sex chromosome introgression barrier is asymmetrical and sex-dependent. Female hybrids showed significant D. albomicans–biased introgression on Muller CD (neo-X excess), while males showed heterosis with excessive (neo-X, D. nasuta Muller CD) genotypes. We used a population genetic model to dissect the interplay of sex chromosome drive, heterospecific pairing incompatibility between the neo-sex chromosomes and unfused Muller CD, neo-Y disadvantage, and neo-X advantage in generating the observed sex chromosome genotypes in females and males. We show that moderate neo-Y disadvantage and D. albomicans specific meiotic drive are required to observe female-specific D. albomicans–biased introgression in this system, together with pairing incompatibility and neo-X advantage. In conclusion, this hybrid swarm between a young species pair sheds light onto the multifaceted roles of neo-sex chromosomes in a sex-dependent asymmetrical introgression barrier at a species boundary.